1. Field of the Invention
The invention relates generally to floating vessels used for supporting offshore drilling operations.
2. Description of the Related Art
Petroleum production often requires the placement of a rig in an offshore location. In shallow waters, the rigs and production facilities can be placed on freestanding offshore platforms. As the water becomes deeper, however, these become impractical, and it is necessary to have a floating platform, or support vessel, upon which the rigs and production facilities can be placed.
One type of deepwater support vessel is the tension leg platform (TLP). The TLP is a buoyant platform that is secured to the seabed using generally vertically-oriented rigid tethers or rods that restrain the platform against vertical and horizontal motion relative to the well in the seabed below. Thus, these platforms have a very short period in response to wave action.
An alternative to the TLP is the deep draft caisson vessel (DDCV). The DDCV is a free floating vessel which is moored to the seabed using flexible tethers so that the vertical and horizontal motion of the vessel is restrained, although not eliminated. Examples of DDCVs are found in U.S. Pat. No. 4,803,321.
Methods for restraining the DDCVs attempt to slow, rather than eliminate, the natural response period of the vessel to wave effects. Current DDCV arrangements “decouple” the vessel from the individual wells being supported so that the wells are not subject to the same induced motions as the vessel. Decoupling is typically accomplished by using buoyant means to make the wells separately freestanding and using flexible hoses to interconnect the vertical risers from the well to the production facilities.
A common variety of DDCV is the type shown in U.S. Pat. No. 4,702,321 that utilizes a long cylindrical structure and is commonly known as a spar. When the vessel is in its installed position, the cylindrical structure exhibits very slow pitch surge and heave motions. Heave motion, however, is not totally eliminated, allowing the structure to bob up and down vertically in the sea. Attempts have been made to add a number of horizontally extending plates along the length of the spar to help the spar be more resistant to heave.
Even with the plates, the spar must be assembled and transported in a horizontal position and then installed by being upended at or near the final site using a large crane that must also be transported to the installation site. As these caisson structures are often around 650 ft. in length, transporting and upending of the structure are risky. Further, it is only after successfully upending and mooring of the structure that components of the rig can be placed atop the spar.
Recently a floating vessel which provides reduced motions and slow natural response periods to heave, but can also be assembled and transported in a vertical, or upright, orientation has been developed. A vessel of this type permits rig components to be placed atop the vessel prior to or during transport.
What is needed is a vessel which can act as a support vehicle for drilling operations on one of these newly developed drilling platforms. There is a necessity for a support vessel that can generate energy and provide extra storage for these new deep-sea drilling vessels, while also providing the same or comparable reduced motions and slow natural response periods to heave, and that can also be assembled and transported in a vertical, or upright, orientation. A vessel of this type permits supplies and equipment to be placed atop and inside the storage areas of the vessel prior to or during transport.